Main propulsion shafting in marine vessels is supported by bearings that maintain the shafting in proper alignment. Bearings located inside the ship's water tight boundary are called line shaft or intermediate bearings. Typically, these bearings are of a steel construction (i.e. steel castings or fabricated of steel plates welded together), conservatively designed, babbitt lined and oil lubricated. Reliability is heavily emphasized in the design of the bearings because there is no redundancy for bearings and a single bearing failure can incapacitate the entire propulsion system.
Traditional intermediate bearings are constructed from a white metal bearing surface centrifugally cast to a steel or iron substrate. They are typically split along the horizontal centre line and are lubricated with oil contained in a sump integral with the base casting. The lubricant is raised from the sump by various means to such a position that the oil can lubricate and cool the bearing. To assist in cooling, the sump volume is often fitted with a cooling coil through which cold water is circulated. In some bearing designs forced lubrication is provided to insure that the bearing will never be starved for oil.
Conventional intermediate bearings are lubricated by a hydrodynamic film of oil and early establishment of this film is essential to avoid boundary operating conditions. For example, very slow speed running with a turning gear, for example, is a very risky operating mode for oil/white metal bearings. Seals are often provided at the ends of the bearings to contain the oil within the bearing housing.
These seals are prone to failure allowing bearing oil to drain into the bilge producing the potential for pollution if no oily bilge separator is fitted to the vessel. More importantly, a bearing with an undetected leak can lose its supply of oil and fail with catastrophic results. In an attempt to avoid this type of problem prior art bearings have been fitted with a sight glass to view the oil level or with thermocouples to measure and transmit bearing temperature to a control room for monitoring.
Intermediate bearings must be properly aligned and raised to such a position that they carry the proper loading. This operation is commonly accomplished by hydraulic jacking of the shaft to the required level and then machining and fitting cast iron chocking pieces between the pedestal and the bearing base. Simplified, bearing chocking procedures have been proposed in the prior art that use a castable epoxy resin poured into the chocking spaces and allowed to harden. Damming is used to contain the chocking compound while the resin is in the liquid state.
In particular, in the prior art hydraulic jack method, a calibrated hydraulic jack is used to determine the actual load supported by a bearing and this actual load is then compared with the desired load. The actual bearing load is determined by placing a hydraulic jack as close to the bearing housing as possible. A dial indicator is located immediately above the jack so as to measure vertical movement of the shaft. The shaft is then raised and lowered in increments, noting the jack load corresponding to each increment of shaft rise.
The hydraulic jack method generates load and influence number errors due to inaccuracies that are inherent in the jacking procedure. In particular, since the jack is not located at the bearing center, the load center in adjacent bearings shift as the shaft is raised, causing hysteresis errors.
Provided traditional bearings are maintained in perfect alignment, they can provide general trouble-free operation. However, the flexible nature of a ship's hull allows settling, twisting and bending such that bearing alignment can change. Additions to the ship's structure, modifications to the moment of inertia of the hull, and different loading conditions can influence the alignment of the shaft supported by bearings. This requires that the bearings be repeatedly re-jacked to ensure shaft alignment. This is a very time consuming and costly process.
The alignment of the intermediate shaft is very important because a metal bearing tends to be very sensitive to edge loading produced as a result of pedestal shifting. Even minor alignment deviations are sufficient to cause an increase in bearing temperature at the edge that can lead to wiping. In the most serious form a wiped bearing can cause shaft seizure, resulting in considerable repair time. These events can occur at inopportune occasions, as severe weather is often responsible for inducing bearing problems.
To deal with the shaft alignment problem, complex pedestal bearings have been produced with the bearing elements mounted in a spherical envelope to permit alignment being taken from the shaft line rather than the pedestal. In particular, the bearing shell is made with a self-aligning feature by providing a spherical or crowned seat at the interface between the bearing shell and the housing. This allows the axis of the bearing shell to align with that of the shaft. The main problem with the spherical type bearing is that any correction in alignment gives rise to displacement of the shaft axis at the ends of the housing where the seals are located. Unless the compensation is very slight the seals are unable to accommodate this eccentricity and will leak. These designs are also costly to produce due to the elaborate additional machining steps involved.
Consequently, there is a need for an intermediate bearing that incorporates an alignment system to enable simple and inexpensive alignment and realignment of the shaft at any time. In addition, there is a need for an intermediate bearing that is essentially selflubricating and capable of operation in oil, water or biodegradable lubricants.